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A COMPARISON BETWEEN ZIRCON-RUTILE THERMOMETRY OF ECLOGITES: IMPLICATIONS FOR TEMPERATURE-TIME EVOLUTION OF THE NORTH QAIDAM UHP TERRANE, CHINA
Eleven MT-UHP eclogites were analyzed integrating Zr-in-rutile thermometry and zircon U-Pb + trace element analysis to constrain the relation between the timing and peak T conditions recorded in the rocks.
Zr-in-rutile thermometry yields a well-constrained temperature range of 651-711 °C (calculated at 30 kbar). On the other hand, Ti-in-zircon thermometry yields a wide temperature range of 610-759 °C. Our results show that the Zr-in-rutile and Ti-in-zircon thermometry have, in general, a good correlation between them, and mostly agree within an error of ± 50°C. However, Zr-in-rutile temperatures yield a tighter temperature range compared with the temperatures obtained from the Ti-in-zircon thermometry.
Zircon U-Pb geochronology yields weighted mean ages of 463-425 Ma, and REE patterns indicate that these ages record the eclogite-facies metamorphism. Inherited zircon cores yield ages of 928-905 Ma, and REE patterns indicate that the eclogites protolith had a magmatic origin. The comparison between the obtained rutile and zircon temperatures with the U-Pb ages show different T-t trends. Temperatures obtained from Zr-in-rutile and zircon U-Pb ages show an apparent increase of ~1°C Ma-1, whereas temperatures obtained from Ti-in-zircon show an apparent decrease of ~11 °C Ma-1. Our results also indicate that Ti-in-zircon temperature are higher than Zr-in-rutile temperatures in older samples, whereas in younger samples, Zr-in-rutile temperatures are higher than the obtained from Ti-in-zircon thermometry.
We interpret that Zr-in-rutile temperatures better represent peak temperatures, whereas Ti-in-zircon record temperatures of different stages along the P-T path. The apparent cooling obtained from Ti-in-zircon and U-Pb ages suggest that temperatures decrease with time.